Antitrust officials investigating four sets of charges against Windows 8 and Windows RT

European antitrust officials have widened their investigation into Microsoft's browser practices to include four different allegations against Windows 8 and Windows RT, a government spokesman said Wednesday.

Antoine Colombani from the European Commission's antitrust agency confirmed that the inquest, originally opened Tuesday to look into Windows 7, has expanded to cover accusations leveled at Windows 8 and Windows RT.

"We will indeed look at these allegations made by third parties in the context of the investigation opened yesterday on Microsoft's compliance with our December 2009 decision," said Colombani in an email reply to questions Wednesday.

Two of the charges raised by outsiders -- which the commission would not name -- had been reported by Reuters, and revolve around APIs (application programming interfaces) in Windows 8 and Windows RT that Microsoft has allegedly withheld from rival browser makers.

Windows RT has garnered the most attention, as Mozilla and Google have expressed frustration over Microsoft's policy of blocking access to special APIs in that edition. Mozilla has claimed that without those APIs, it cannot build a browser competitive with Internet Explorer 10 (IE10) on tablets.

The Windows 8 APIs relate to what Colombani called "non-default browsers in Windows 8." Although he did not elaborate, Colombani may have been referring to a Windows 8 practice that prevents a browser's Metro version from running unless its desktop edition has been designated by the user as the system default.

Microsoft sets IE10 as the default browser in Windows 8. Users can change the default, but they may not know where the option is located.

Additionally, said Colombani, the commission is scrutinizing two other allegations.

One relates to Microsoft's practice of providing co-marketing funds to OEMs, or original equipment manufacturers, such as Dell, Hewlett-Packard and Lenovo.

"Third parties allege that Microsoft is acting in violation of the browser commitments by discouraging computer manufacturers (OEMs) by means of payments or withholding payments under marketing programmes from setting third-party browsers as default in Windows 8," Colombani said.

According to the agreement that Microsoft signed in December 2009, it cannot set conditions that forbid OEMs from installing other browsers on their PCs. Nor can the company retaliate against an OEM that distributes third-party browsers like Chrome, Firefox, Opera or Safari.

The fourth and final accusation claims that Microsoft does not display the browser choice screen on devices running Windows RT.

The browser choice ballot was the centerpiece of the 2009 settlement, which required Microsoft to give users easier access to rivals' Web browsers.

According to the agreement, which expires at the end of 2014, the browser choice screen must appear in all copies of Windows, including Windows 8. Previously, the commission had declined to say whether it believed Windows RT, the offshoot designed for ARM processor-powered tablets, was covered by the deal.

It appears that EU regulators are now prepared to ponder exactly that.

Although the commission refused to name the parties that reported the alleged violations, Mozilla has been vocal about the browser choice screen and Windows RT. In a May blog post, Mozilla's chief counsel, Harvey Anderson, argued that Windows RT should be held to the same requirements as Windows 8.

"If Windows on ARM is simply another version of Windows on new hardware, it also runs afoul of the EC browser choice commitments," Anderson said at the time.

But a browser choice screen in Windows RT may be hard to populate. No other browser maker has publicly committed to develop for the new operating system, although Google and Mozilla are working on Windows 8 versions of Chrome and Firefox that will include both desktop and Metro versions. The Metro-style Chrome and Firefox browsers should also run on Windows RT, albeit slower than IE10 if Microsoft is not forced to unblock access to Windows RT's APIs.

Microsoft's latest confrontation with the commission is reminiscent of the one that preceded Windows 7's launch in 2009.

In June 2009, near the end of Windows 7's development, Microsoft said it would ship the operating system without Internet Explorer 8 (IE8) to European customers to sidestep the commission pressure and keep Windows 7's launch on track.

For a time, Microsoft stuck to that plan -- it was going to call the European edition Windows 7 E -- but only weeks later it caved and agreed to a browser choice screen.

Windows 8 is much closer to completion than Windows 7 was three years ago when the commission pushed for a browser ballot. It would be very difficult, if not impossible, for Microsoft to modify Windows 8, Windows RT or both, and still make the Oct. 26 on-sale date for the former.

Microsoft has declined to comment on the commission's investigation beyond the statement it issued Tuesday.